Optical discs including CDs (Compact Discs), DVDs (Digital Versatile Discs) and MDs (Mini Discs) have been widespread recently. Optical disc systems for recording and reproduction of information with these discs have so-called pickups therein.
Optical pickups can be classified to discrete type pickups and hybrid type pickups. Discrete type pickups are equipped with an objective lens, beam splitter, collimator lens, cylindrical lens, semiconductor laser and PD (Photo Detector). Hybrid type pickups are equipped with an objective lens and an integrated optical device incorporating all elements other than the objective lens.
In discrete type pickups, the semiconductor laser is held in a package called CAN package. In hybrid type pickups, the integrated optical device is held in a package called flat package. Herein below, a CAN package holding a semiconductor laser will be called a CAN package light emitting device, and a flat package holding an integrated optical element is called a flat package light emitting device.
FIG. 17 shows a configuration of a CAN package light emitting device. As shown in FIG. 17, the CAN package 102 houses a semiconductor laser 101 bonded on a sub mount 106 as well as a monitor PD (not shown). For bonding the semiconductor laser 101 to the sub mount 106, a solder of Sn, SnPd, AuSn, In, or the like, is used. The structure having the semiconductor laser 101 bonded on the sub mount 106 is herein called LOS. For bonding a heat sink 105 to the sub mount 106, Ag paste is used. For bonding the package 102 to the monitor PD, Ag paste is used. The Ag paste can be regarded as an organic adhesive containing Ag powder.
FIG. 18 shows a configuration of a flat package light emitting device. As shown in FIG. 18, the flat package 112 houses a sub mount 115 supporting a semiconductor laser 111 bonded thereon, mirror prism 116 and PDIC (Photo Detector Integrated Circuit) 117. For bonding the semiconductor laser 111 to the sub mount 115, a solder of Sn, SnPb, AuSn, In, or the like, is used. The flat package light emitting device shown here has the LOS structure having the semiconductor laser 111 and the sub mount 115 bonded together.
For bonding the sub mount 115 to the holder shell 113, Ag paste is used. For bonding the PDIC 117 to the holder shell 113, Ag paste is used. As mentioned above, the Ag paste can be regarded as an organic adhesive containing Ag powder. For bonding the mirror prism 116 to the holder shell 113, an epoxy organic adhesive utilizing ultraviolet-setting, thermosetting or combination of ultraviolet and heat is used. For bonding seal glass 114 to the holder shell 113, an epoxy organic adhesive, for example, is used in general.
As mentioned above, in both CAN package light emitting devices and flat package light emitting devices, organic adhesives are used for bonding parts to the packages. Organic adhesives can be classified to metamorphic acrylate adhesives, epoxy adhesives, silicone adhesives, and so on, depending upon the base polymer. In terms of the form of curing reaction, they can be classified to ultraviolet-setting type adhesives that start curing with irradiation of ultraviolet rays; thermosetting type adhesives that start curing with application of heat; adhesives that start curing by reaction with oxygen; and so on. To determine the form of curing reaction, so-called curing agents are added to organic adhesives.
As mentioned above, adhesives are generally classified in terms of the base polymer and form of curing reaction. However, even in a common class, adhesives are usually different in physical and chemical properties. To control these properties, various substances are added in form of compounds.
For example, so-called catalysts are added to assist reactions. Furthermore, other substances are added to adhesives to control various properties such as viscosity before setting, hardness after setting, interfacial chemical coupling forms, and so on.
Moreover, fine particles of metals such as Ag, Au, Cu, and so forth, are added to some adhesives to control the heat conductivity and electrical conductivity. They are widely known as Ag paste, Au paste and Cu paste. Even in this case, various substances are added to control the physical property such as viscosity, hardness, dielectric constant, and so on.
Adhesives are combined for dedicated use depending upon the natures of parts to be bonded, intended properties after their combination, environment of intended use of the entire device composed of the parts. Combinations of adhesives are know-how of manufacturers and developers of adhesives, and details of components of adhesives are not opened to users in most cases.
Characteristics deterioration of semiconductor lasers caused by adhesives will be discussed below. Heretofore known are (1) characteristics deterioration by diffusion of heat and (2) characteristics deterioration by generation of gas as characteristics deterioration of semiconductor lasers caused by adhesives.
(1) Characteristics Deterioration by Diffusion of Heat
In case a semiconductor laser is in contact with an adhesive containing metal, it is known that characteristics of the semiconductor laser deteriorate due to the metal contained in the adhesive and diffusing into the semiconductors.
For example, in case the semiconductor laser is bonded to a package by an adhesive containing Na, the following process of deterioration is known. First, Na receives heat energy and diffuses into the semiconductor laser. As the diffusion progresses, Na reaches the proximity of the active layer of the laser, and deteriorates the emission efficiency (because it functions as an absorber). Otherwise, Na destroys the p/n bonding interface, and invites changes in wavelength and deterioration of emission efficiency.
Also when the metal contains in the adhesive is Zn or Pd, it has been known that semiconductor lasers are subject to characteristics deterioration. For example, characteristic deterioration caused by an adhesive containing Zn is a well-known phenomenon in semiconductor lasers made of GaInP compound semiconductors for the band of 650 nm. It causes shifting to longer wavelengths by destruction of natural superlattices (which is sometimes used positively) or degradation of emission efficiency, which finally results in an increase of the drive current.
In view of these problems, it has been proposed to use a solder material as the adhesive by forming a diffusion stop layer between the adhesive and the semiconductor laser. In a structure where the adhesive is not in direct contact with the semiconductor, Ag paste is used frequently. In this case, the phenomenon of undesirable diffusion into the semiconductor does not occur. Used as the solder material is a metal such as Sn, SnPb, In, AgSn or AuSn. These metals used as the solder material have a high purity. Especially when the semiconductor laser does not include defects (in particular, penetrating dislocation), a material having a smaller diffusion constant is used. Furthermore, a material suppressing diffusion of the solder is frequently used as the semiconductor bonding surface. For example, Pt is frequently used for this purpose (for example, as the p-side electrode for the band of 780 nm or p-side electrode for the band of 650 nm). By using this type of structure, it is possible to prevent deterioration of the operative characteristics of the semiconductor laser by diffusion of the metal elements into the semiconductor.
FIG. 19 shows a semiconductor laser having a diffusion stop layer. As shown in FIG. 19, an adhesive layer 122 of a solder, Au layer 123 bringing about mutual diffusion with the solder, diffusion stop layer 124 for preventing diffusion of the solder, and Ti layer 125 for bonding of the semiconductor laser 126 and the metal are sequentially stacked on a sub mount 121. The semiconductor laser 126 is bonded on the Ti layer 125.
(2) Characteristics Deterioration by Generation of Gas
As referred to above, organic adhesives are made of mixing many compounds, and volatile substances may be included in the components. In some cases, vaporization of the base polymer itself may occur depending upon the curing condition. Although the packages are different in sealing capability, they define essentially sealed spaces. Therefore, gas of volatile substances reaches and adheres to the outer wall of the package or other members mounted in the package, as well as the lid, and invites deterioration of characteristics. Especially in the flat package light emitting device shown in FIG. 18, organic adhesives causing characteristics deterioration are widely used.
For example, in the flat package light emitting device shown in FIG. 18, the seal glass 114 has the function of transmitting light. If the volatile components adhere and cure on the light-transmitting portion thereof of the seal glass 114 and seriously degrades the transmittance, it invites deterioration of the light path and the distribution of the emission intensity.
For example, in case of a flat package light emitting device to be mounted in an optical disc system, wavefront aberration will significantly increase, and it will disturb reproduction of signal from a disc and writing of signal on the disc.
Therefore, adhesives that will generate less amounts of gas are usually chosen as adhesives. To avoid characteristics deterioration, the use of epoxy adhesives less volatile than acrylic adhesives, for example, is regarded preferable. Among a common kind of base polymers, the use of ultrasonic-setting type polymers is considered more preferable than the use of thermosetting type polymers.
Vaporization from adhesive is caused by heat in general. Vaporizing amount n can be expressed in terms of the capacity V, temperature T in the package, and the intrinsic saturation vapor pressure of the volatile components.
That is, the vaporizing amount n can be expressed asn=PV/RT  (1)Regarding adhesion to other members, it is assumed that the vaporizing gas solidifies on these members by thermal reaction, i.e. because the temperature of the members is lower than the temperature of the gas.
Organic resins shaped to desired configurations, either thermosetting or thermoplastic, or regardless of their processing methods, are widely used as lead frame packages for housing integrated optical elements. These packages are composed of shaped organic resins and metal portions. It has been believed heretofore that the organic resins used in those packages do not cause the same deterioration as the above-mentioned deterioration by adhesives.
Recently, shorter wavelengths and higher optical outputs have been required with semiconductor lasers. For example, according to the next-generation high-density optical discs (Blu-ray Discs), wavelength of the laser light used for recording and reproduction is 405 nm. As a light source for emitting laser light of this wavelength, the use of a group III nitride compound semiconductor laser is under examination.
However, the Inventor prepared a packaged light emitting device housing a 405 nm semiconductor laser, and repeated various experiments with this light emitting device. As a result, the Inventor has found that the light emitting device is subjected to fluctuation of the drive current, fluctuation of the optical output, changes of the beam profile and deviation of the optical axis.